In a variety of scientific, industrial, and medically related applications, it can be desirable to transfer energy or power across some type of boundary. For example, one or more devices that require power can be located within the confines of a closed system in which it may be difficult and/or undesirable to include a substantial and/or long term source of power. It can also be undesirable to repeatedly enter the closed system for a variety of reasons. In these cases, a power source external to the closed system and some feasible means of transferring power from the external source to one or more internal devices without direct electrical conduction can be preferable.
One example of a closed system is the human body. In several medically related and scientific applications, a variety of prosthetic and other devices that require power may be surgically implanted within various portions of the body. Examples of such devices include a synthetic replacement heart, a circulatory blood pump or ventricular assist device (VAD), and the like. With respect to the human body, complications associated with repeated surgical entry make replaceable internal power sources impractical. Likewise, the risk of infection and/or dislodgment makes direct electrical linkages between external power supplies and implanted devices undesirable.
Accordingly, transcutaneous energy transfer (TET) systems are employed to transfer energy from outside the body to inside the body in order to provide power to one or more implanted devices from an external power source. TET systems use an inductive link to transfer power without puncturing the skin. Thus, the possibility of infection is reduced while comfort and convenience for patients is increased.
TET devices typically include an external primary coil and circuitry, along with an implanted secondary coil and circuitry that are separated by intervening layers of tissue. The primary coil is designed to induce alternating current in the subcutaneously placed secondary coil, typically for transformation to direct current to power an implanted device. TET devices therefore also typically include electrical circuits for periodically providing appropriate alternating current to the primary coil. These circuits typically receive their power from an external power source.
As a result of the power demands of exemplary implanted medical devices, such as a VAD, the TET primary coil must frequently be coupled to the implanted secondary coil to supply power from the external power source. Accordingly, it is desirable for the external power source and primary coil to have a simple, automated operation that can easily be used by a patient, nurse, or doctor. In prior art implementations, this is accomplished via an “always on” configuration, where the primary coil constantly transmits power when connected to an external power source.
However, such prior art implementations can have several drawbacks. For example, if a patient or practitioner decouples the primary coil from the implanted secondary coil for any reason (e.g., to reposition a patient, change clothes, etc.), the primary coil can inadvertently transfer power into surrounding objects. A typical example may be a patient who removes a primary coil from their skin in order to change clothing. The patient may place the primary coil on, for example, a stainless steel table in a hospital. The primary coil, however, may continue to transmit power as if it were coupled to the secondary coil. This continued transfer of power can result in undesirable heating of the metal table or other objects placed on the table.
In addition, the external power source may be a limited capacity source, such as a battery pack. In such a case, the continued transfer of power when the primary coil is decoupled from the secondary coil is extremely inefficient and undesirably depletes the battery pack charge.
Hence, there is a need for a method of controlling power output from a primary coil in a TET system in order to avoid inadvertent transmission of power when the primary coil is not coupled to an implanted secondary coil.